Activity of sdAb19 on CD4 down-regulation induced by HIV-1 Nef alleles. CEM T cells were cotransfected with plasmids for expression of nef alleles in combination with increasing amounts of the plasmid for expression of sdAb19 (1:3 and 1:8 ratios) or an irrelevant sdAb (1:0 ratio). Twenty-four hours later, cells were stained with PE-Cy5–conjugated anti-CD4 at 4°C, and cell surface expression was measured by flow cytometry. Results are expressed as the percentage of the MFI determined in GFP-positive cells relative to that determined in GFP-negative cells. Values are the means of 3 independent experiments. Error bars represent 1 SD from the mean.